The present invention relates to a data transmission method in a cellular communications system comprising in each cell at least one base station communicating with mobile stations within its coverage area, in which method extra known reference information is added to a transmit signal.
In a cellular communications system the quality and parameters of a radio channel may vary greatly as a function of time and location. When a mobile station is stationary, the radio channel is nearly invariant. When the station is on the move, the radio channel varies as a function of time on account of the mobility of the station and the multipath propagation of the radio signal. In such a constantly changing situation, it is difficult to achieve reliable data transmission. Channel coding, signal interleaving and coherent data transmission may be employed to improve transmission quality. In certain cases, transmission quality may also be improved by employing multilevel signals, such as a multilevel orthogonal or biorthogonal signal, in transmitting the data signal.
The prerequisite for applying coherent reception is a carrier phase estimate for the receive signal, and also an estimate of the radio channel if optimum performance is desired. Channel estimation may be carried out by transmitting either a separate pilot signal, known reference symbols, or a mere data signal.
The pilot signal method employs a separate continuous narrowband or wideband signal to generate a phase reference estimate. EIA/TIA Interim Standard; Mobile Station--Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TIA/EIA/IS-95, July 1993, specifies a method employing a separate pilot channel on which a predetermined data unmodulated signal is sent. Adding a separate pilot channel to the transmission takes up part of the available transmission capacity, renders the transmitter and receiver constructions complex, and may necessitate expansion of the requisite transmission bandwidth. Therefore, a separate pilot channel is unfeasible, particularly in the transmission direction from mobile to base (uplink). In the CDMA multiple access method, however, a pilot channel may have utility in the transmission direction from base to mobile (downlink).
Channel estimation by means of a data signal may be implemented either by means of decision feedback or without feedback. In these methods, channel estimation does not necessitate expanding the required transmission bandwidth. Estimation making use of a data signal is, however, poorly suited to the transmission path environment of cellular systems. When decision feedback is employed, the problem resides in the unreliability of the decisions applied. On the other hand, without feedback the required channel estimation circuit is difficult to implement, particularly when multilevel signals are employed.
Prior art methods based on the use of known reference symbols are attended by the drawback that they do not take account of the state of the radio channel. A prior art method for using reference symbols is described in Fuyun Ling, Coherent Detection with Reference Symbol Based Channel Estimation for Direct Sequence CDMA Uplink Communication, IEEE Vehicular Technology Conference 1993 incorporated herein by reference. This publication describes a method in which a mobile station transmits reference information at regular intervals to a base station. Thus the channel is always measured at a constant frequency by means of reference symbols fully independently of the rate of change of the channel. The symbol transmission rate, i.e. the number of symbols sent in a time unit, is dependent on how rapid measurement of the changing channel is desired and what amount of transmission capacity (power or bandwidth) is to be allocated for these known symbols. The object is to achieve a sufficient channel measurement accuracy while utilizing a minimum of transmission capacity. At a constant frequency, the solution is always a compromise between these two factors, and the result cannot be an ideal solution in conditions of varying channel measurement needs.
Another known method is disclosed in Finnish Patent Application 940148, in which a mobile station transmits known reference symbols with frame symbols corresponding to those employed by the base station in sending power adjustment commands to the mobile station. Also in this method, the amount of reference information is not dependent on the rate of change of the channel but on the amount of power adjustment information sent by the base station, and this is not directly proportional to the rate of change of the channel.